Clearance of excess cholesterol from cells by high density lipoproteins (HDL) is facilitated by the interaction of HDL apolipoprotein with cell-surface binding sites or receptors (Mendez et al., J. Clin. Invest. 94:1698-1705, 1994), such as ABCA1 (Oram and Yokoyama, J. Lipid Res. 37:2473-2491, 1996). ABCA1 is a member of the ATP binding cassette transporter family (Dean and Chimini., J. Lipid Res. 42:1007-1017, 2001) and is expressed by many cell types (Langmann et al., Biochem. Biophys. Res. Commun. 257:29-33, 1999). Mutations in the ABCA1 transporter lead to Tangier disease, which is characterized by the accumulation of excess cellular cholesterol, low levels of HDL and an increased risk for cardiovascular disease (Rust et al., Nat. Genet. 22:352-355, 1999; Bodzioch et al., Nat. Genet. 22:347-351, 1999; Brooks-Wilson et al., Nat. Genet. 22:336-345, 1999; Remaley et al., Proc. Natl. Acad. Sci. USA 96:12685-12690, 1999; and Lawn et al., J. Clin. Invest. 104:R25-R31, 1999). Fibroblasts from Tangier disease patients are defective in the initial step of cholesterol and phospholipid efflux to extracellular apolipoproteins (Francis et al., J. Clin. Invest. 96:78-87, 1995 and Remaley et al., Arterioscler. Thromb. Vasc. Biol. 17:1813-1821, 1997).
Research has demonstrated an inverse correlation between the occurrence of atherosclerosis events and levels of HDL and its most abundant protein constituent, apolipoprotein A-I (apoA-I) (Panagotopulos et al., J. Biol. Chem. 277:39477-39484, 2002). ApoA-I has been shown to promote lipid efflux from ABCA1-transfected cells (Wang et al., J. Biol. Chem. 275:33053-33058, 2000; Hamon et al., Nat. Cell Biol. 2:399-406, 2000; and Remaley et al., Biochem. Biophys. Res. Commun. 280:818-823, 2001). However, the nature of the interaction between apoA-I and ABCA1 is not fully understood. Several other exchangeable-type apolipoproteins have also been shown to efflux lipid from ABCA1-transfected cells (Remaley et al., Biochem. Biophys. Res. Commun. 280:818-823, 2001). Although the exchangeable-type apolipoproteins do not share a similar primary amino acid sequence, they all contain amphipathic helices, a structural motif known to facilitate the interaction of proteins with lipids (Segrest et al., J. Lipid Res. 33:141-166, 1992 and Anantharamaiah et al., J. Biol. Chem. 260:10248-10255, 1985). Animal experiments have shown that intravenous injections of apoA-I or its variant, apoA-I Milano (which has a cysteine substitution at position 173 for arginine), produced significant regression of atherosclerosis (Rubin et al., Nature 353:265-267, 1991 and Nissen et al., JAMA 290:2292-2300, 2003). These results make apoA-1, or derivatives thereof, attractive as potential therapeutic compounds in the treatment and prevention of atherosclerosis.
Short synthetic peptide mimics of apolipoproteins have been used as a model for studying physical and biological properties of apolipoproteins (see, e.g., Fukushima et al., J. Am. Chem. Soc. 101:3703-3704, 1980; Kanellis et al., J. Biol. Chem. 255:11464-11472, 1980; and U.S. Pat. Nos. 4,643,988, and 6,376,464). These include, for instance, single helices taken from native apolipoproteins, synthetic amphipathic alpha helices (Kanellis et al., J. Biol. Chem. 255:11464-11472, 1980), and derivatives thereof. Examples of short synthetic amphipathic helical peptides have been shown to promote lipid efflux and inhibit atherosclerosis (Garber et al, J. Lipid Res. 42:545-552, 2001; Navab et al., Circulation 105:290-292, 2002; and U.S. Pat. No. 6,156,727). However, while some of these peptides exhibit beneficial effects in preventing atherosclerosis, they are also potentially cytotoxic (Remaley et al., J. Lipid Res. 44:828-836, 2003). It is believed that the cytotoxicity is caused by non-specific, ABCA1-independent lipid efflux from cells (Remaley et al., J. Lipid Res. 44:828-836, 2003). Therefore, there exists a need for non-cytotoxic synthetic peptide mimics of apolipoproteins that promote specific lipid efflux from cells by an ABCA1-dependent pathway for use in the treatment and prevention of cardiovascular diseases, such as atherosclerosis.